基于体积法对黄土细沟侵蚀沿程分布模拟的研究

A VOLUMETRIC METHOD BASED STUDY ON DISTRIBUTION OF EROSION ALONG RILLS ON LOESS SLOPE

通过土槽冲刷试验结合体积法估算不同流量(2、4、8 L min-1)和坡度(5°、10°、15°、20°、25°)条件下,细沟侵蚀体积及其分布规律,进而研究黄土细沟侵蚀过程及坡度、流量对其的影响。结果表明:细沟侵蚀过程不是沿其均一恒定的,累积侵蚀泥沙量及含沙量均随着细沟的增长近似呈指数增加,且这个趋势在陡坡和大流量下更为显著。坡度和流量的增大均能造成累积侵蚀泥沙量及含沙量的增加,即导致细沟侵蚀程度加重,但是流量对于侵蚀的影响权重远大于坡度。且实测的侵蚀泥沙总量及水流含沙量与体积法测得累积侵蚀泥沙量及含沙量的对比验证了本试验体积法估算细沟侵蚀总量及沿程分布规律的准确性及实用性,其结果为细沟侵蚀动态过程研究及预测提供了理论依据。

Rill erosion is a serious environmental problem threatening the agricultural production safety and sustainable societal development in the Loess Plateau, China. It is, therefore, very important and urgent to quantify dynamic process of the rill erosion. To that end, an indoor simulation experiment was carried out the scouring and backfilling method. Erosion process was analyzed through observing variation of the shape of rills and measuring volume of erosion along the rill segment by segment. Cumulative amount of erosion along the rills was worked out by refilling the separated rill segments with water to their original elevation. The sediment concentration was calculated by dividing the cumulative amount of erosion at each segment by the total volume of the water flow during the erosion period. A typical silty-loam soil was sampled from the Loess Plateau of China （109° 19′ 23″ E, 36° 51′ 30″ N） and used as experimental soil in this study. The soil contains 16.31% clay （〈0.005 mm）, 61.35% silt （0.005 to 0.05 mm）, and 22.34% sand （〉0.05 mm）. The soil was air-dried and then passed through an 8 mm sieve. In order to a complete continuous erosion process, an experimental trough, 3.0 wide and 12.0 m long, filled with loess, was used. In the trough six experimental rills, 0.1 m wide and 12 m long, were built to simulate well-develped rills. The trough was put at horizontal position for soil packing. At the bottom, the trough was densely packed with a 5cm thick layer of local clay soil, made compact with a hammer to simulate the plow pan layer, 15 g cm-3 in bulk density. The upper 20 cm of the trough was packed with loess in layers, about 5 cm each, to make the soil even in bulk density, ranging approximately between 1 150 to 1 200 kg m-3. The soil surface was raked to make it as rough as natural condition. Cumulative volume of erosion of a rill was the sum of erosion volumes of the eleven rill segments （0～0.5 m, 0.5～1 m, 1～2 m, 2～3 m, 3～4 m, 4～5 m, 5～6 m, 6～7 m, 7～8 m, 8～10 m, 10～12 m） of a rill. Total amount of erosion sediment was measured volumetrically by a collecting vat at the outlet of each rill during the experiment. The experiment was designed to have five slope gradients, i.e. 5？, 10？, 15？, 20？ and 25？ for the trough, three flow rates, i.e. 2, 4 and 8 L min-1 and three replicates. Results show that the process of rill erosion was not constant along a rill, with cumulative rill erosion and sediment concentration increasing exponentially and margin of the increase declining with rill length till the extreme in the end, which means that cumulative amount of erosion and sediment concentration increases rapidly at the initial segments of the rills, and the increase rate （the slope of the curve） attenuates gradually to approach zero along the rill. Flow rate and slope gradient were the two major factors affecting rill erosion, but the former seemed to have more influence than the latter. The comparison of the cumulative amount of erosion measured with the volumetric method with the result of direct measuring at the rill outlet validated the accuracy of the former, and the comparison of the sediment concentration measured with the water backfilling method with the result of direct measuring demonstrated that the former was quite accurate in and applicable to investigating distribution of erosion sediment along a rill. The findings of this study may serve as certain basis for consummating the research on soil rill erosion process, assessing sediment concentration from rill erosion and establishing a prediction model for farmland rill erosion.